This thesis gives an overview of which factors the size of the LNG fuel tank is dependent on and how these may be used to estimate the optimal size of the LNG fuel tank for a shuttle tanker in the North Sea.

From the 1970s shuttle tankers have been used in the North Sea to transport oil from offshore oil fields to onshore refineries. The shuttle tankers are a good alternative to pipelines in remote locations, deep water and harsh climates. In recent years, the use of shuttle tankers has spread, and there has been a greater focus on the industry having to take responsibility for a greener shipping of the oil. This has resulted in the first LNG driven shuttle tanker being built at the SHI shipyard in South Korea and is to be delivered to Teekay in 2019.

The E-shuttle tankers are planned with hybrid electric propulsion with the latest generation 4- stroke dual fuel machinery and an advanced gas system for handling VOC and LNG. This new shuttle tanker will operate on both LNG as primary fuel, and a mixture of LNG and VOCs as secondary fuel, in addition to batteries, and as a result the annual emissions may be reduced by up to 42 percent compared to conventional shuttle tankers.

However, with new classes, there are also new challenges, one of which is the LNG fuel tank. The objective of this master thesis is to give an overview of which factors the size of the LNG fuel tank is dependent on and how these may be used to estimate the optimal size of the LNG fuel tank for a shuttle tanker in the North Sea.

Important factors include the shuttle tankers operational profile, utilizing of VOC and challenges around LNG as ship fuel. These are elaborated in the thesis, and methods to take these factors into account when designing the fuel tank are discussed, like simulation and optimization.

In this thesis a discrete-event simulation model was constructed based on the operational profile of the shuttle tanker Nansen Spirit in 2017. The model includes 70 trips in the North Sea, from Heidrun in the North to Bilbao in the South. In order to take account of the weather conditions along the routes, historical weather data in the North Sea has been downloaded and implemented in the model. Fictive bunker prices were also included in the model, which was based on historical data and studies including the one done by Danish Maritime Authority(2012).

The results from the simulation model were discussed and it was concluded that the optimal size of the LNG fuel tank would be 2000 cubic meters. With this tank size the shuttle tanker will have a fairly flexible operational profile, and the use of LNG will account for approximately 80% of the total fuel consumption.

In conclusion, some recommendations for further work are presented. These include processing and implementing AIS-data, adding stochastically modelled weather conditions to the simulation model, research expenses like OPEX and CAPEX connected to the LNG fuel tank and assess if these can be implanted in an optimisation cost-function.